The present invention relates to a helicopter rotor, in particular, an FH (FLEX-HUB) rotor.
Of the abovementioned types of rotor, one is known on which the foot of each blade is clamped between the arms of an essentially U-shaped coupling fork extending essentially radially outwards from a center hub. The said hub essentially consists of a metal plate, usually made of titanium, comprising a center portion from which flat flexible arms, each with a center aperture, extend radially outwards. The said aperture is engaged by the relative fork the closed inner part of which, facing the said hub, is provided with a rod connected to the center portion of the said plate by means of an elastomer ball support. A further elastomer support is inserted to connect each fork to the relative flexible arm portion which closes outwardly the said aperture on the arm so as to form a bridge extending between the arms on the relative fork and in the space between the foot of the relative blade and the inner end of the fork.
On the FH rotor just described, the said flat plate and, in particular, each of the flexible radial pieces on it are subjected, in use, to both centrifugal load and lift bending moments. The said flat plate is therefore forced to operate under severe working conditions in that, when in use, it is subjected not only to tensile stress due to centrifugal force but also to continual bending stress in directions varying continually in cycles. In use, the plate is therefore subjected to severe fatigue load which may soon cause it to collapse if it is not made perfectly using extremely high-cost engineering methods. In addition to the aforementioned manufacturing problems, the said plate also involves a number of design difficulties posed by the need for a fine compromise between a high degree of elasticity on the plate in the air and the rigidity required on the ground to withstand the static loads transmitted by the blades.